ActiveMQ scheduling ignored in 5.15.8 - scheduled-tasks

Question

I have been using the scheduling feature in ActiveMQ to delay a message.
However, when switching from version 5.9.0 to 5.15.8, suddenly the delay setting is ignored. Does anyone have a clue about why?
The ActiveMQ broker is defined as
<broker xmlns="http://activemq.apache.org/schema/core" brokerName="myBroker" dataDirectory="${activemq.data}" schedulerSupport="true">
in 5.9.0 and as
<broker xmlns="http://activemq.apache.org/schema/core" brokerName="myBroker" dataDirectory="${activemq.data}" schedulerSupport="true">
in 5.15.8.
The delay is, in my Java code, set via
message.setLongProperty(ScheduledMessage.AMQ_SCHEDULED_DELAY, 120000);
As said, this works perfectly fine (i.e., messages delivered after two minutes) in version 5.9.0, but is totally ignored (i.e., messages delivered immediately) in 5.15.8. Both versions are started using the same script, just changing the relevant path.
Diffing the activemq.xml files I can't see anything I think is significant:
[servers]# diff apache-activemq-5.15.8/conf/activemq.xml apache-activemq-5.9.0/conf/activemq.xml
32c32
< <bean id="logQuery" class="io.fabric8.insight.log.log4j.Log4jLogQuery"
---
> <bean id="logQuery" class="org.fusesource.insight.log.log4j.Log4jLogQuery"
84c84
< <!--<persistenceAdapter>
---
> <persistenceAdapter>
86,93c86,87
< </persistenceAdapter>-->
< <persistenceAdapter>
< <jdbcPersistenceAdapter dataSource="#mssql-ds" lockDataSource="#mssql-ds-lock" lockKeepAlivePeriod="5000">
< <locker>
< <lease-database-locker lockAcquireSleepInterval="10000"/>
< </locker>
< </jdbcPersistenceAdapter>
< </persistenceAdapter>
---
> </persistenceAdapter>
>